Pretty sure those sites break in Edge and Brave as well...
Performance. That makes the sabotage so insidious!
https://tech.co/news/google-slowed-youtube-firefox-edge-2019...
Firefox is the only browser we use and we watch Youtube every day. If it broke, I'd know about it or hear about it.
It never breaks.
If they are close to the edge cases of the spec, just not testing on Firefox will break it. By negligence, rather than malice.